Elastic-plastic fracture assessment of CNT-reinforced epoxy/nanocomposite specimens weakened by U-shaped notches under mixed mode loading

For the first time, mixed mode I/II fracture of U-notched semicircular bend (USCB) specimens fabricated from a ductile carbon nanotube (CNT)-reinforced epoxy/nanocomposite material is investigated experimentally and theoretically. The U-notched nanocomposite specimens are prepared with different notch rotation angles and various notch tip radii. Mixed mode I/II fracture tests are conducted on these specimens by applying the three-point bending loading in order to measure their load-carrying capacities (LCCs) and fracture initiation angles (FIAs). For theoretical fracture estimations, the well-known U-notched maximum tangential stress (UMTS) criterion is employed in conjunction with the reformulated Equivalent Material Concept (EMC). To compare the experimental and theoretical results, the effective notch stress intensity factor (ENSIF) and the notch mode mixity parameter are defined and computed for the notched nanocomposite specimens tested. It is found that the EMC-UMTS combined criterion can provide good predictions of the mixed mode notch fracture toughness, as well as the FIAs, without performing any nonlinear analyses. The micromechanical failure processes of the nanocomposite specimens are also investigated using the pictures taken by the scanning electron microscope (SEM) from the fracture surfaces. It is shown that the SEM pictures confirm well the fracture results obtained from the macro-mechanical analyses.